A Polymer electrolyte membrane or proton exchange membrane (PEM) fuel cells having intrinsic benefits and a wide range of applications due to their relatively low operating temperatures (room temperature to approximately 80.degree. C.). The active portion of a PEM is a membrane sandwiched between an anode and a cathode layer. Fuel containing hydrogen is passed over the anode and oxygen (air) is passed over the cathode. The reactants, through the electrolyte (the membrane), react indirectly with each other generating an electrical voltage between the cathode and anode. Typical electrical potentials of PEM cells can range from 0.5 to 0.9 volts; the higher the voltage the greater the electrochemical efficiency. However, at lower cell voltages, the current density is higher but there is eventually a peak value in power density for a given set of operating conditions.
Multiple cells are combined by stacking, interconnecting individual cells in electrical series. The voltage generated by the cell stack is effectively the sum of the individual cell voltages. There are designs that use multiple cells in parallel or in a combination series parallel connection. Separator plates (bipolar plates) are inserted between the cells to separate the anode reactant of one cell from the cathode reactant of the next cell. To provide hydrogen to the anode and oxygen to the cathode without mixing, a system of fluid distribution and seals is required.
Cell voltage monitoring (CVM) systems for fuel cell stacks provide important cell voltage state-of-health information to the fuel cell system controller. Typically, the CVM system provides real time feedback and can initiate a system alarm or shutdown if a significant variation in cell voltage distribution is measured, or cell voltages are below a specific threshold value. Low cell voltages can be the result of poor operating conditions causing cell flooding or drying, or can indicate hydrogen starvation if a cell voltage becomes negative. These situations can adversely affect the performance and durability of the fuel cell stack.
Typically, cell voltage monitoring (CVM) pick-up assemblies are attached to fuel cell stacks after the stack is assembled. However, a number of drawbacks exist with this design. For example, the distance between each CVM pick-up must be very precise since tolerance stack up over the length of a fuel cell stack of many cells can result in significant CVM pick-up misalignment. Also, as the stack expands and contracts in length during normal operation, the CVM pick-up assembly must also expand and contract to maintain good cell-to-cell contact. Furthermore, the CVM pick-ups must maintain proper preload against the surface of the cell to minimize contact resistance and ensure good electrical contact. Various stack designs also have variable cell pitches so it is difficult to have a “generic” CVM system.
Thus, there is a need for an improved CVM pick-up assembly for use with fuel cell stacks.